"Linux has a large amount of device drivers for hardware not supported on FreeBSD, especially USB devices. Not rarely, such drivers have been written based on information derived by protocol sniffing, reverse engineering and the like. This makes the code highly undocumented, and renders the porting effort extremely error prone. To help with this task, I decided to start working on an emulation layer that would let us recompile the linux source code on FreeBSD, and provide a sufficiently complete emulation of the kernel APIs so that device drivers (or at least certain classes) could be used without modifications to their source code."

Probably by using the (unfortunately not so well known) fact that NO free software license places restrictions on how you USE the code (as opposed to distributing the code). Rest assured that you are free to link code licensed under incompatible licenses to your heart's content as long as you don't distribute the linked binary.

Now, the author is distributing binary .ko modules, which are presumably built from GPL Linux driver code linked with GPL Linux compatibility code, but also linked with BSD kernel header files. Kernel modules are a grey area in the definition of "derivative work." Many people believe that kernel modules, even distributed separately from the core kernel, are inherently derivative works of the kernel whose headers were linked in at build time. Others think this should be an exception for pragmatic reasons.

Regardless, if you download the sources and build the modules yourself, there is no grey area. This is perfectly legal and, in fact, a right protected by all OSI-approved licenses.

>The two major categories of free software license are copyleft and non-copyleft . Copyleft licenses such as the GNU GPL insist that modified versions of the program must be free software as well. Non-copyleft licenses do not insist on this. We recommend copyleft, because it protects freedom for all users, but non-copylefted software can still be free software, and useful to the free software community.